WingSail Rake and Gybe Angle Question

Discussion in 'Hydrodynamics and Aerodynamics' started by Erwan, Jul 18, 2011.

  1. Erwan
    Joined: Oct 2005
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    Location: France

    Erwan Senior Member

    Hi Everybody,

    Working on my wing project on a scale 1 blue-print, for the hound, I am a bit embarrassed with the mast rake as it seems to be the main variable for the gybe angle.

    Considering an A-Cat exemple, the stays or shrouds are 50cm behind the mast rotation pin.
    With the hound at 18 feet (550cm) hight, and a NACA18-like wing section for element 1:
    With no rake,(mast is vertical) the max angle at the hound is around 38°
    With maximum rake (when the hound is brought at the vertical of the stays)
    the angle can reach 65° at the hound.
    At max rake the angle should reach 90° but the thickness of the wing section offsets a part of the gain.

    The middle rake position bring the angle around 46°.

    I would like to have a little idea of which amount of maximum rake is acceptable for "structural integrity"

    And if a C-Cat specialist can provide a little idea of the max angle achieved on a C-Cat wing at the hound, it will be perfect as C-Cat like A-Cat have no front sail or asymetric spinnaker, and therefore share the same gybe angle issue, the difference would be probably speed difference leading to different apparent wind angle.

    Thanks in advance

    Regards to all

    Erwan
     
  2. Erwan
    Joined: Oct 2005
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    Erwan Senior Member

    Sorry Dough, I just realized I could have written it on the Wingsail Technology forum.

    Feel free to change it if you want

    Apologizes

    Erwan
     
  3. Gary Baigent
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    Location: auckland nz

    Gary Baigent Senior Member

    If I'm reading you correctly, Ewan, your question relates to mast rotation angles; if not, then ignore what I'm saying here.
    A thinner 12% wing would make difference. On my setups I can rotate the wing to 90 degrees either side ... but to do so the leeward shroud has to be loose enough for it not to bind on the side of the wing. Some of the C Class designs don't have parrot beak forestay/shroud attachment points (as I have) but have sunken rails set into and around the hounds area that allow the main wing element to rotate without binding, yet still have tight stays. But I've found the less complex, simple parrot beak works fine. Also my shrouds are not savagely tight ... but I have running stays to control mast movement in waves. The beak has to protrude forward of the mast leading edge. Raking the mast depends on your helm balance - set up with a few degrees and test sail.
     

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  4. Erwan
    Joined: Oct 2005
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    Erwan Senior Member

    Thanks Gary,

    For aero reason, I cannot downsize the wing section to 12%; I am afraid the only solution is maximum rake, because with a wing you need stay & shrouds tension which contributes to the plateform stiffness.

    With a structural mast @ 7.5% of the wing section, I just hope the wing aera in front of the mast will balance a part the excess aft rake.

    Flat modern A-Cat hull shape are no too senstitive to rake change, but with too much rake it becomes difficult to tack in strong wind. That is the point, otherwise, I have to change the daggerboard position.

    Nice B-Cat, an historical boat I guess;

    Best regards

    Erwan
     
  5. Gary Baigent
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    Location: auckland nz

    Gary Baigent Senior Member

    You could try shifting the pivot point at mast base to the trailing edge base, that will put a little more mass forward.
    The B Class sail rig is from an old catamaran named Boadicea, which used to be top B Class in Auckland until the Tornadoes arrived - but the wing mast in the jpeg is a newer/now also old, lighter and smaller chord (460mm) version; Boadicea's original was three times the weight and around 480mm chord - but the boat in picture is not a cat but a foil trimaran, recently defunct, although the rig still lives.
     
  6. philSweet
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    philSweet Senior Member

    Can you post a sketch? I'm thinking you're missing something in the 3D geometry. But I'm probably misreading your question. By gybe angle, do you mean the angle between boat centerline and wing cordline?
     

  7. Erwan
    Joined: Oct 2005
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    Location: France

    Erwan Senior Member

    Thanks Phil,

    Yes you are right, sorry my formulation is misleading, I should have mentionned the angle of the wing cordline relative to the boat centerline at the hound level. The gybe angle is a by-product of this former one.

    As I do not want to rule out downwind sailing on 2 hulls in light winds, I consider that the wing/boat angle at the hound is the limiting factor for gybe angle in low wind.

    Now considering the new downwind sailing approach "Wild Thing on the Wire" On the pics and video of the Australian Nat it seems that the sail is quite open in these conditions ?? to be confirmed.

    So it can be a limiting factor in the breeze as well, that is why I pay a lot of attention to this pb.

    Cheers

    Erwan
     
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